Scraped Surface Crystallizers

Our scraped surface continuous crystallizer consists of multiple double pipe elements, generally with 6, 8, 10, or 12 inch nominal diameter inner pipes and larger diameter outer pipes. The annular space between the inner pipe and outer pipe is filled with cooling fluid. Each inner pipe contains a rotating scraper blade element which mixes the process fluid flowing through the inner pipe, and removes deposits which form on the inner pipe wall as cooling occurs.

In effect our scraped surface continuous crystallizer is a heat exchanger, but quite an unusual one, because it generally performs as a cooling crystallizer. Heat transfer occurs across the inner pipe wall, with cold fluid outside, and process fluid inside. As cooling occurs, crystals tend to form on the inner pipe wall. The scraper blades rotate on the inner pipe wall and remove these deposits which would inhibit heat transfer. The majority of the crystallization takes place in the bulk of the fluid, as opposed to the wall, thus allowing growth of easily separable crystals.

A typical commercial sized installation will consist of several double pipe elements, each with a length of 20 to 40 feet, connected in series. This provides a long thin flow path which promotes a close approach to plug flow, which is very important in many crystallizations.

Advantages of Scraped Surface Continuous Crystallizers over other methods of Crystallization

• Smaller equipment, which generally means less expensive installations, less floor space needed, less operator labor, and no duplication of instrumentation, pipe, etc.

• Better process control, less upsets of hazardous or expensive materials and less peak utility demand.

• Modular design allows for easy expansion with growth in demand.

• Simple, self contained construction with minimum instrumentation and auxiliaries such as condensers, vacuum systems, etc.

• May be run for extended periods between hot washings whereas many shell and tube exchangers would plug up in minutes.

• May be run at much higher process fluid-coolant temperature differences than could shell and tube equipment without serious fouling or plugging.

• May be used over an extremely wide temperature range (-75 C to +100 C).

• May be used with high percentages of solids (as high as 65% solids as slurry).

• High viscosities are not a problem (has been used with mother liquor viscosities of 10,000 cp or higher).

• Flow pattern in once-through operation closely approaches plug flow so conversion from batch operation is easy and virtually any desired time/temperature pattern is possible.

• In small capacity cases, a scraped surface crystallizer will be very inexpensive. This is also true in cases where, for much larger installations, vacuum crystallization may seem most attractive.

Dewaxing of Lubricating Oils

Dewaxing lubricating oil represents the largest use of our scraped surface continuous crystallizers.  Wax has the same boiling point range as lubricating oil fractions, but has a much higher freezing point.  Therefore, cooling crystallization is a very effective way to separate the two materials.

Many of the lube oil processing plants are quite large and require many scraped surface continuous crystallizers, often with a number of units in a series.  Larger plants usually require several parallel trains of crystallizers.

Armstrong/Chemtec is frequently involved in the process design to optimize heat transfer, residence time and pressure drops, as well as determining the optimal use of refrigeration.  A primary goal in designing scraped surface crystallizers for dewaxing is to maximize the time on stream between turnarounds.

Crystallization of Organic Compounds

Many organic mixtures may be separated by cooling crystallization.   In simple terms, cooling crystallization means that a mixture of organic chemicals is partially crystallized by reduction in temperature, without removal of any of the components by evaporation.

Crystallization is a one-way process; the heat is removed, crystals are formed, and the mixture of solids and liquids are then separated.  Many crystallizations take place at near ambient temperature so there is little heat up or cool down required to get the right conditions for the separation to start. 

In the great majority of crystallizations, the crystals which form are 100% pure material, as opposed to something only slightly richer than the feed material as in distillation.  With crystallization, it is not necessary to repeatedly melt and refreeze to obtain high purity.  Minor impurities can be present in cooling crystallization but these can be removed easily.

Viscous Liquid Cooling

Highly viscous fluids do not present problems to the scraped surface heat exchanger but may make other heat exchangers totally inoperable.  Nearly all liquids show an increase in viscosity as they cool.  With a highly viscous fluid, this effect may present significant resistance to heat transfer at the cold wall and thereby greatly increase the required surface area.  Scraped surface exchangers greatly reduce the resistance at the cold wall by constantly disrupting (reducing) the wall boundary layer and exposing the wall to warmer, lower viscosity fluid.

Pilot Testing Program

Armstrong/Chemtec offers a pilot testing program to help you determine whether crystallization is the best method of separation for your specific application.  This enables you to evaluate the process before scale up.   A number of pilot units are available on a rental basis.  The sizes of these units range from a 4" diameter by 4' long jacketed section to 12" by 10'.  These units are as small as can be used to gather reliable information on the process.  The rental units are normally either carbon steel throughout or SS316 on the process side.

Over the many years, Armstrong/Chemtec engineers have assisted with dozens of piloting programs of organic compounds using our scraped surface crystallizers. As a result, we know how a given compound will react and can often save a great deal of time and effort by avoiding tests that will not scale up.  Since many various tests are usually required to complete piloting, it is recommended to conduct the testing at the client's plant, where separation equipment, feed stock including variations, etc., are available.

A significant amount of information can be learned from piloting:

• Heat transfer characteristics

• Some idea of crystal size

• Crystal habit and size distribution

• Best cooling cycle and residence time

• Methods of separation to be used

• Crystal purity

• Best solvent or magma ratios

• Color pickup